Related fields include thin-film devices, particularly the fabrication of resistive thin films including ternary metal nitrides.
Ternary metal nitrides are nitrides of the form (m)(m) (m′)Nx, where x>0 and (m) and (m′) are different metallic elements. Some ternary metal nitrides exhibit properties desirable in a thin-film resistor, such as the ability to maintain a constant resistance over a range of operating conditions, or the ability to tune the resistance over a range of values by manipulating the thickness or composition.
For example, a type of non-volatile memory known as “resistive-switching memory” or “ReRAM” includes a variable-resistance (VR) layer that is reversibly switchable between two or more stable values of resistance. When initially fabricated, the VR layer is dielectric, with high resistance. When the device is complete, a “forming operation” applies voltage across the VR layer until it becomes more conductive. For example, electrical defects, such as metal particles or oxygen vacancies, may be present in the bulk of the VR layer, in an adjacent “source” layer, or both. The application of the forming voltage organizes some of the defects into a conductive filament, through which electrons can tunnel across the VR layer. After that, a much lower applied voltage can break or disperse part of the filament to raise the VR layer's resistance to its high resistance state (HRS) or restore the broken filament to return the VR layer's resistance to its low resistance state (LRS). In many types of ReRAM, the breaking and restoring of the filament involves only the movement, creation, or destruction of electrical defects without requiring a phase change of the bulk material.
One mode of ReRAM failure is “over-forming,” the formation of a filament that is too thick or dense to break under normal operating conditions (essentially, an irreversible breakdown of the VR layer). This may occur either during the forming operation that first creates the filament or later during a switching event to restore the filament (e.g., if the current is overly high or has a transient spike, or if an overlarge number of defects are accumulated at the switching point). Once over-formed, the VR layer cannot be returned to the LRS; i.e., the memory cell can no longer be switched and its logic state cannot be rewritten.
One approach to prevent over-forming of a ReRAM cell is to connect a non-switching resistor in series with the VR layer. When the non-switching resistor is part of the film stack that constitutes the ReRAM cell, it may be referred to as an “embedded resistor” (ER). With an ER in the cell, any voltage applied to the cell's electrodes is divided between the VR and ER layers, rather than being wholly applied to the VR layer, and the current through the VR layer is limited to a level that can break a filament, or restore a breakable filament, but not create an unbreakable filament.
An ER layer in an operating ReRAM cell may have a sheet resistivity of 10−1-103 Ω-cm. Thicknesses may range from 2-50 nm; a common range is 2-10 nm. Desirable qualities in a ReRAM ER layer include a constant resistance under all operating conditions for the life of the device. In some devices, it is preferred that the ER resistance is unchanged by the forming operation, or by heating to a temperature between 500-1000 C for a time between 10 seconds and 10 minutes to anneal other components such as diodes. Throughout fabrication and operation, the ER layer preferably does not interact with other layers in the stack in a way that compromises memory-cell performance. For example, its component materials should not diffuse into other layers, and it should not scavenge oxygen from metal-oxide VR layers.
Ternary metal nitrides with (m)=aluminum (Al), molybdenum (Mo), tantalum (Ta), titanium (Ti), vanadium (V), or tungsten (W) and (m′)=boron (B), silicon (Si), or Al (if (m)≠Al) are promising candidate materials for ER layers. However, their use has been limited by the historical difficulty of depositing them in fully-compounded form. Some atomic layer deposition (ALD) and chemical vapor deposition (CVD) precursors for these materials are slow to react with nitrogen, requiring inconveniently long process times to form the nitride. As for sputtering (either from a nitride target or from a metal target in a nitrogen atmosphere), compound targets of these elements have been observed to deposit unwanted large particles, deposit a variable composition across a substrate due to sputter-angle sensitivity, or both.
Therefore, a need exists for a reliable, cost-effective way to deposit layers of ternary metal nitrides such as AlBNx, AlSiNx, MoAlNx, MoBNx, MoSiNx, TaAlNx, TaBNx, TaSiNx, TiAlNx, TiBNx, TiSiNx, VAlNx, VBNx, VSiNx, WAlNx, WBNx, and WSiNx.